The dehydrogenation has been described in various prior arts. U.S. Pat. No. 3,256,355 relates to the dehydrogenation of ethylbenzene to make styrene. In said prior art, referring to the stream leaving the ethylbenzene dehydrogenation reactor, the latent heat of condensation of the contained steam is used to heat the reboilers of distillation columns in the overall process. Said stream is firstly washed with hot water to remove tars, then compressed and sent to the reboilers.
U.S. Pat. No. 4,288,234 describes an ethylbenzene dehydrogenation wherein the stream leaving the ethylbenzene dehydrogenation reactor is introduced in a cooling zone containing one or more cooling steps and a compression step. The remaining gases, essentially hydrogen, are washed with ethylbenzene and then polyethylbenzene to remove aromatics.
U.S. Pat. No. 4,628,136 describes an ethylbenzene dehydrogenation wherein the stream leaving the ethylbenzene dehydrogenation reactor is introduced in a conventional cooling zone where are recovered (i) a gaseous phase (essentially hydrogen), (ii) an organic phase (ethylbenzene and styrene) and (iii) an aqueous phase. Said aqueous phase is further mixed with fresh ethylbenzene then is vaporized while condensing the reflux of the ethylbenzene/styrene distillation column and then sent to the dehydrogenation catalyst.
U.S. Pat. No. 6,388,155 relates to a process for the production of styrene monomer from ethylbenzene comprising the steps of:
a) catalytically dehydrogenating said ethylbenzene in the presence of steam thereby catalytically producing a dehydrogenation effluent gas containing unreacted ethylbenzene and lighter components and styrene monomer and heavier components;
b) scrubbing said effluent gas with reflux to remove at least a portion of said styrene monomer and heavier components from said effluent gas;
c) condensing said scrubbed effluent gas thereby producing a liquid organic dehydrogenation mixture, an aqueous phase and a gaseous phase; and
d) using a portion of said liquid organic dehydrogenation mixture as said reflux for said step b) of scrubbing.
In the bottoms of said scrubber of step b) an aqueous phase and an organic phase are recovered, said aqueous phase is mixed with aqueous phase recovered at step c) and said organic phase is fed to a distillation column for separation of the ethylbenzene and styrene monomer.
In said processes there are water streams containing small amounts of various polymeric materials and condensed heavy components. The polymeric materials are essentially coming from polymerization of divinylbenzene with styrene. The condensed heavy components are produced in small amounts in the dehydrogenation process. By simplification, in the following description and claims, these polymeric materials and condensed heavy components are often called polyaromatics. When said aqueous phase containing polyaromatics goes through vessels, heat exchangers, boilers etc. . . . there is a serious risk of fouling of said equipment by the polyaromatics. Moreover because these polyaromatics might be originated from divinylbenzene, a cross-linking might occur.
It has been discovered that introduction of an aromatic component, advantageously ethylbenzene or benzene or toluene or mixture thereof cause the migration of polyaromatics to an organic phase. Said organic phase is then separated from the aqueous phase in a decanter. This leads to a clean aqueous phase leaving the decanter. “Clean aqueous phase” means that said aqueous phase leaving the decanter will not induce fouling or random polymerizations in the vessels, pipes and any piece of equipment.